Astronomers from Boston University have released images revealing details of Mercury's unseen side.

Mercury is a mysterious world, the closest planet to the Sun. It is about the same size as our own Moon and on the surface it is similar. Its landscape is crater-strewn like our Moon but inside Mercury has a large core of iron, unlike our Moon's tiny iron core.

Mercury from Mariner 10

It has been visited only once by the Mariner 10 spacecraft in 1974.

"More than a quarter-century ago, the Mariner 10 spacecraft flew past Mercury and for the first and only time transmitted satellite-based photos of half of the surface of the planet closest to the Sun," says Jeffrey Baumgardner, senior research associate in the Center for Space
Physics.

"Capturing similar images from a ground-based telescope represents a significant milestone in advanced instrumentation," he adds.

The images, taken two years ago at the Mt. Wilson Observatory in California, reveal surface markings similar to the bright craters and dark regions found on the Moon. Image processing show never-before-seen-portions of Mercury.

Challenging

Photographing Mercury is challenging because of the planet's proximity to the Sun. Mercury only has a few good viewing times, before sunrise or shortly after sunset.

Opportunities to photograph Mercury from space are also limited because light sensitive equipment, such as the Hubble Space Telescope, are not allowed to look at objects close to the Sun, such as Mercury or Venus.

This restriction has been established to avoid the possibility of an accidental pointing error causing too much light to fall upon an instrument.

"The observations were made shortly after sunrise before the Sun's heating of the atmosphere distorted the images captured by the telescope," says Michael Mendillo, professor of astronomy at Boston University.

The Boston University team plans to make additional observations of Mercury later this year.

"Mercury has a thin atmosphere created by the ejection of atoms from its surface, a process that also occurs on our Moon," Mendillo explained. One of the chemical elements in Mercury's atmosphere is sodium, a gas somewhat easy to detect because it reflects sunlight very efficiently.

"We hope to try our first sodium detection experiments this fall," Baumgardner said. "But that will first involve building a more sensitive detector system."